In this work a chipless RFID tag based on a surface wave impedance (SIW) and a Van-Atta array is presented and assessed. The Van-Atta array it is aimed to reflect the impinging electromagnetic wave on every angle of incidence. The information is encoded in a the compact set of SIW resonators. To improve the detection capabilities and improve the signal to noise ratio, a quantile regression method to enhance the system performance has been considered. A tag prototype has been designed, fabricated, and experimental assessed. The obtained results are quite promising and they demonstrate the potentialities of the proposed chipless tag.